The manufacture of optical waveguide fibers has long passed from an early, primarily experimental stage to a fully commercial stage in which a growing number of customers' transmission needs are being satisfied over short and long distances and at various wavelengths corresponding to visible as well as to invisible radiation. The manufacture of commercial fiber typically is based on silica glass technology and involves drawing from a massive body or preform having a cross-sectional refractive index profile as designed for effective guiding of one or several radiation modes.
With respect to most currently used optical fiber, optical waveguide structure can be described in terms of a higer-index core portion which is surrounded by a lower-index portion such as, typically, a glass cladding. At the core-cladding interface there may be a relatively abrupt change in refractive index; alternatively, and especially in the case of fibers designed for the transmission of a plurality of modes, refractive index may decrease gradually towards a fiber surface. A refractive index difference between core and cladding typically results from the addition of one or several suitably chosen dopants or additives to otherwise essentially pure silica; e.g., the addition of boron or fluorine results in a lowered (cladding) refractive index, and the addition of aluminum, germanium, phosphorus, or titanium produces an increased (core) refractive index. With respect to fluorine, germanium, and phosphorus see, e.g., J. Irven et al., "Long-wavelength Performance of Optical Fibers Co-doped with Fluorine", Electronics Letters, Vol. 17 (1981), pp. 3-5.
Recently, more elaborate refractive index profiles have been disclosed; for example, U.S. Pat. No. 4,435,040, issued Mar. 6, 1984 to L. G. Cohen et al. discloses so-called W-profile or double-clad optical fibers.
Considerable progress has been made in the development of methods for the manufacture of optical waveguide fiber preforms, and a number of such methods have been found capable of producing preforms from which low-loss fibers can be drawn. One such method is described, e.g., in U.S. Pat. No. 4,217,027, issued Aug. 12, 1980 to J. B. MacChesney et al. Still, and such progress notwithstanding, development efforts continue, e.g., towards further reducing intrinsic loss, such reduction being in the interest of lengthening the distance over which signals can be transmitted without amplification or regeneration.